This invention relates to an electrostatic erasing abrasion-proof coating and method for forming the same.
Abrasion-proof coatings are formed over surfaces which has a tendency to take scratches due to external rubbing actions. The surface of glass plates which may be used for transmitting light therethrough is a typical example of such a surface. Contact image sensor, which have been recently developed, are suitable for use in compact facsimile machines, copying machines or the like. The image sensor makes direct contact with an original and scans the surface of the original by moving relative to this.
An example of the contact image sensor is illustrated in FIG. 1. The sensor comprises a glass substrate 1, a photosensitive semiconductor device 2, a transparent protective layer 3, an adhesive layer 4, an ITO film 5 and a glass pane 6. An original bearing an image to be sensed is placed in contact with the external surface of the glass pane 6. The ITO film, which is a transparent conductive film, is grounded for the purpose of canceling out electrostatic charges collected on the contact surface of the pane 6 due to rubbing action between the original 9 and the glass pane 6. In case of treatment of usual papers, the size of scratches may be of the order of 1 micron meter or less so that the performance of the sensor is not substantially deteriorated by the scratches. However, if a staple is held to a paper to be telefaxed, the paper may give scratches of substantial size which degrade the quality of the transmission. Furthermore, the use of the ITO film for canceling out static electricity increases the size and the production cost of the device.
It is therefore an object of the invention to provide an excellent abrasion-proof coatings and methods for forming the same which produce no static electricity on the coating even when rubbing action takes place thereon.
In order to accomplish the above and other objects, it is proposed to coat a surface with carbon films in different deposition conditions in order that the external surface of the coating has a higher degree of hardness for providing an abrasion-proof surface and that the carbon coating includes an inner layer whose resistivity is comparatively low (conducting) to extinguish the influence of static electricity. This structure can be realized by inverting the polarity of the pair of electrodes, between which direct or high frequency electric energy is supplied, an object to be coated being mounted on one of the electrodes. When the electrode supporting the object is supplied with high frequency energy (that is to say, the electrode functions as the cathode), the hardness of carbon material becomes high. On the other hand, when the electrode supporting the object is grounded (i.e., the electrode functions as an anode), the hardness becomes low but the conductivity thereof becomes high. By letting the surface be a cathode, carbon material being deposited is eliminated due to bombardment of positive ions such as hydrogen ions, where the elimination rate of soft carbon material is higher than that of hard carbon material.
According to a preferred embodiment of the present invention, the energy band gap of carbon product for forming the external abrasion-proof surface of the coating is not lower than 1.0 eV, preferably 1.5 to 5.5 eV: the Vickers hardness is not lower than 500 Kg/mm2, preferably not lower than 2000 Kg/mm2, ideally not lower than 6500 Kg/mm2, at the external surface of carbon coatings: the resistivity ranges from 1010 to 1015 ohm centimeter: and the thermal conductivity of the product is not lower than 2.5 W/cm deg, preferably 4.0 to 6.0 W/cm deg. When used for thermal heads or contact image sensor which are frequently subjected to rubbing action, the smooth, hard and static erasing surface of the carbon coating is very suitable. The carbon coating includes an inner layer region having a low resistivity. The Vickers hardness and the resistivity of the inner layer region are not higher than 1000 Kg/mm2, preferably 500 to 700 Kg/mm2, and not higher than 1012 ohm centimeter, preferably 1xc3x97102 to 1xc3x97106 ohm centimeter. The inner layer region has lower Vickers hardness and higher conductivity than the external surface.